It is known a method of obtaining a self-supporting GaN crystal as follows. That is, a GaN crystal is grown on a sapphire substrate or a GaN template to obtain a composite substrate, and the GaN crystal is separated from the sapphire substrate by, for example, laser lift-off method to obtain the self-supporting GaN crystal.
According to this method, the composite substrate may be warped due to a difference of thermal expansion between the sapphire substrate and gallium nitride film, resulting in warpage or cracks during the separation of the gallium nitride film by the laser lift-off method.
According to patent document 1, a nitride single crystal is formed on a sapphire substrate by vapor phase process (HVPE, MOCVD, MBE) in a chamber, and the lasser lift-off process is then performed continuously on the site in the same chamber at the same temperature, for eliminating the influences of the warpage of the composite substrate and preventing the generation of the cracks.
Further, the applicant tried to provide a gallium nitride layer on a sapphire substrate while a layer containing inclusions, is provided in an initial stage of the formation of the gallium nitride layer, so that the cracks could be successfully prevented during the laser lift-off process (Patent document 2).